1. Field of the Invention
The present invention relates in general to a dynamic random access memory (DRAM) having a trench-type capacitor. More particularly, the present invention relates to a DRAM having a guard ring and processes of fabricating the same.
2. Description of the Related Art
A memory device stores data at specified voltage levels in an array of the cells. Conventionally, the voltage levels represent that the data is either a logical xe2x80x9c1xe2x80x9d or a logical xe2x80x9c0xe2x80x9d. In dynamic random access memory devices, for example, the cells store the data as a charge on capacitor. When the data is read from the memory device, sense amplifiers detect the level of charge stored on a particular capacitor so as to produce a logical xe2x80x9c1xe2x80x9d or logical xe2x80x9c0xe2x80x9doutput based on the stored charge.
Typically, in a DRAM fabricating process, a guard ring is formed on the peripheral circuit area to prevent electrostatic discharge from damaging the memory device.
FIGS. 1A through 1E and FIG. 3 illustrate a process flow of the fabrication of a DRAM having a guard ring according to a prior art.
Referring now to FIG. 1A and FIG. 3, a cross-sectional view of semiconductor substrate 10 having a memory array area I and a guard ring area II is schematically shown. The semiconductor substrate 10 is selectively etched to respectively form a first trench 12 and a second trench 14 on the memory array area I and on the guard ring area II. These trenches 12, 14 both have length-width dimensions of about 0.45xcexcmxc3x970.2 xcexcm. Next, a first thermal oxide layer 16 having dopants and a second thermal oxide layer 18 having dopants are respectively formed on the sidewalls of the first trench 12 and the second trench 14 by in-situ doping thermal oxidation.
Next, as shown in FIG. 1B, an organic material 20, for example photoresist, is coated on the semiconductor substrate 10 and filled into the first trench 12 and second trench 14 by a spin coating method.
Referring to FIG. 1C, the organic material 20 is partially removed by reactive ion etching (RIE) to leave an organic material 20a within the first trench 12 and an organic material 20b within the second trench 14.
As shown in FIGS. 1C and 1D, a wet etching step is used to remove the exposed thermal oxide layer 16 and the exposed thermal oxide layer 18 so as to leave a thermal oxide 16a and a thermal oxide 18a, while the organic material 20a and the organic material 20b are used as etching masks.
Next, referring to FIG. 1E, the remaining organic material 20a and the remaining organic material 20b are removed. Then, a thermal treatment is utilized so that the dopants of the thermal oxide 16a and the thermal oxide 18a are diffused into the adjacent semiconductor substrate 10 to respectively form a first doped plate 22 and a second doped plate 24. These plates are called xe2x80x9cburied platesxe2x80x9d by those skilled in the art, and serve as storage electrodes of a trench-type capacitor. Next, a first doped strap 26 is formed on the upper surface of the semiconductor substrate 10 around the first trench 12. At the same time, a second doped strap 28 is formed on the upper surface of the semiconductor substrate 10 around the second trench 14. These straps are called xe2x80x9cburied strapsxe2x80x9dby those skilled in the art, and connect the drain of MOS transistor formed in the subsequent step. Then, referring to FIG. 3, a photoresist mask is formed on the space S as shown by FIG. 3. Next, an implantation step is performed to form a N-well 30 so that the second doped strap 28 and the second doped plate 24 are electrically connected.
According to the method described above, formation of the N-well 30 is required so that the second doped strap 28 and the second doped plate 24 are electrically connected. As a result, space S having 0.65 xcexcm width is needed. Therefore, wafer area is wasted. Moreover, the additional photo-mask for N-well 30 adds cost to the manufacturing process.
In view of the above disadvantages, an object of the present invention is to provide a DRAM having a guard ring so as to economize the wafer area and to reduce the manufacturing cost by alteration of the trench dimension on the guard ring area.
In order to achieve the above object, a DRAM having a guard ring is provided, the DRAM comprising: a semiconductor substrate having a memory array area and a guard ring area; a first trench disposed within said semiconductor substrate on said memory array area; a second trench disposed within said semiconductor substrate on said guard ring area; a first doped strap disposed on the upper surface of said semiconductor substrate around said first trench; a second doped strap disposed on the upper surface of said semiconductor substrate around said second trench; a first doped plate disposed on said semiconductor substrate around the bottom of said first trench, and separated from said first doped strap by a predetermined distance; and a second doped plate disposed on said semiconductor substrate around the bottom of said second trench, and connected to said second doped strap.
Furthermore, in an embodiment of the present invention, the first trench and the second trench respectively have length-width dimensions of about 0.45 xcexcmxc3x970.2 xcexcm and 0.2 xcexcmxc3x970.2 xcexcm.
Furthermore, in another embodiment of the present invention, the first doped strap and the second doped strap comprise n-type dopants. Moreover, the first doped plate and the second doped plate can comprise n-type dopants.
Furthermore, in order to achieve the above object, a process of fabricating a DRAM having a guard ring is provided, the process comprising the steps of: (a) providing a semiconductor substrate having a memory array area and a guard ring area; (b) selectively etching said semiconductor substrate to form a first trench on said memory array area and a second trench on said guard ring area respectively, wherein the dimension of said second trench is smaller than that of said first trench; (c) respectively forming a first thermal oxide layer containing dopants and a second thermal oxide layer containing dopants on the sidewalls of said first trench and said second trench; (d) partially removing said first and said second thermal oxide layers so that the distance between the top portion of said first trench and the remaining first thermal oxide layer is larger than that between the top portion of said second trench and the remaining second thermal oxide layer; (e) performing a thermal treatment to diffuse said dopants of said first and said second thermal oxide layers into adjacent semiconductor substrate to form a first doped strap and a second doped strap respectively; (f) respectively forming a first doped plate and a second doped plate on the upper surfaces of said semiconductor substrate around said first trench and said second trench so that said first doped strap is separated from said first doped plate by a predetermined distance, and said second doped strap is connected to said second doped plate.